A typical image sensor senses light by converting impinging photons into electrons that are integrated (collected) in the sensor pixels. After completion of integration cycle charge is converted into a voltage that is supplied to the output terminals of the sensor. The charge to voltage conversion is accomplished either directly in the sensor pixels, such as in the Active Pixel CMOS image sensors, or remotely, off the sensing area, in charge conversion amplifiers. The key element of every charge conversion amplifier is the charge detection node. As charge is transferred onto the node its potential changes in proportion to the amount of transferred charge and this represents signal. The charge detection node is typically connected to a gate of a suitable MOS transistor that serves as a first stage of the amplifier. The charge detection node is also provided with a reset transistor that removes charge from the node after sensing.
There are many charge detection node and amplifier designs known in the literature. The most popular structure is the Floating Diffusion (FD) architecture. The detail description of such systems can be found, for example, in the book: “Solid-State Imaging with Charge-Coupled Devices” by Albert J. P. Theuwissen pp. 76–79 that was published in 1995 by Kluwer Academic Publishers.
The performance of any charge detection system can be evaluated according to the following main criteria: the charge conversion factor, dynamic range, noise floor, reset feed-through, and linearity. The charge conversion factor is determined by the overall detection node capacitance that also includes the node parasitic capacitances. It is thus desirable to minimize the parasitic capacitances and maximize the charge conversion factor. The Dynamic Range (DR) of the node is determined by the ratio of maximum charge handling capacity to the noise floor. It is desirable to minimize the noise floor in order to maximize the DR. The FD charge detection node has to be reset after sensing of charge. The reset is typically accomplished by turning on a reset transistor that is connected to the node. The reset transistor, however, causes a reset feed through that is introduced to the node through a capacitive coupling from the gate of the reset transistor. The reset feed through consumes a portion of the DR, so it is advantageous to minimize the reset feed through. A significant portion of the reset feed through is also caused by charge spilling back from the reset transistor channel onto the node when the transistor is turned off. This charge spill back contributes noise, so it is advantageous to minimize it.